This invention relates to acoustic logging of boreholes and, more particularly, to a method for determining the true dip angle and direction of a subsurface formation fracture intersecting a deviated borehole.
In U.S. Pat. Nos. 3,668,619 to Dennis; 3,369,626 to Zemanek, Jr.; 3,718,204 to Groenendyke; and 3,728,672 Dennis et al, there are disclosed methods of and apparatus for scanning the walls of a borehole with acoustic energy. In these patents a borehole televiewer logging tool employs a transmitter and a receiver of acoustic energy which are rotated within the borehole. The transmitter is cyclically energized to provide a beam of acoustic energy pulses for scanning the walls of the borehole. Reflected acoustic pulses are received by the receiver between transmitted acoustic pulses and are converted to reflection signals for recording on an electron beam display device. A sweep signal is generated each time the acoustic energy beam is rotated through a 360.degree. scanning pattern. Such sweep signal is applied to the horizontal deflection plates of the display device to sweep an electron beam horizontally across the face of the display device. The reflection signals are applied to the Z-axis of the display device to intensity modulate the electron beam as the beam is swept across the face of the display device to provide a picture which is a function of the time or distance from the transmitter and receiver to the wall of the borehole and of the acoustic reflectivity of the borehole wall.
In the above described patents the combination of transducer rotation along with vertical movement of the borehole televiewer logging tool along the length of the borehole results in a continuous spiral of the borehole wall being scanned. The resulting display is a picture of the acoustic reflectivity of the material forming the walls of the borehole at different depth points. This scanning of the borehole wall permits the determination of the actual configuration of the borehole. In addition, it permits the determination of anomalies which may exist at different depths in the borehole. For example, these anomalies may be a fault or a fracture in the formations traversed by the borehole.
In U.S. Pat. No. 4,780,857 to Lyle et al, there is disclosed an acoustic logging method employing a borehole televiewer logging system for determining fracture dip angle and dip direction from the location of the center of a fracture with respect to a vertical borehole and the peak amplitude and phase angle of the recorded borehole signal at the fracture location. However, if the borehole intersecting the fracture is deviated away from the vertical, it becomes necessary to correct the relative dip angle and dip direction for such borehole deviation effects in order to arrive at the true dip angle and true dip direction.
Others have described methods for correcting measured dip angle and dip direction for deviated boreholes. For example, in an article entitled "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tapes" by Moran, Caufleau, Miller and Timmons published in the Journal of Petroleum Technology, July 1962, pgs. 771-782, there is described a method for correcting microresistivity dipmeter determined dip angle and dip direction. The microresistivity dipmeter employs three electrical pad devices spaced uniformly about a logging tool for the making of microresistivity measurements. From such measurements, three microresistivity dipmeter curves are recorded and used to identify depth displacements from which dip angle and dip direction are determined for deviated borehole effects. Since the Moran et al transformation is for an electrical pad type device, their determination of the transformation contains certain angle variables referenced to a single one of the pad electrodes. Since the borehole televiewer acoustic logging system does not employ electrical type measurements and pads, the Moran et al transformation may not be applied to transform the borehole televiewer relative dip angle and dip direction to true dip angle and dip direction.
In European patent application 0 232 561 to Rambow entitled "Borehole Televiewer Dipmeter" there is described a method for correcting relative dip angle and dip direction for borehole deviation by using both the borehole deviation and the earth's magnetic inclination in the vicinity of the borehole. This information is used to compute true dip angle and dip direction of the fracture by using Euler angle techniques, i.e., a predetermined series of matrix rotations.
It is a specific object of the present invention to provide a method that transforms relative dip angle and dip direction of a subsurface fracture intersecting a borehole into true dip angle and dip direction that has not heretofore been applied through prior art techniques to borehole televiewer acoustic logging data.